The present invention relates to an optical system and a visual display apparatus. More particularly, the present invention relates to an optical system which is usable as either an ocular optical system or an imaging optical system, and which provides high resolution with minimal field curvature and comatic aberration even when the field angle is wide and the pupil diameter is large. The present invention also relates to a visual display apparatus which uses such an optical system.
A Schmidt system, which is well known as an objective for astronomical telescopes, is generally arranged such that an aspherical lens which is close to a plane-parallel plate is placed at the spherical center of a concave mirror to correct spherical aberration, and a diaphragm is disposed at the spherical center to correct coma and astigmatism. Concentric optical systems, which are represented by the Schmidt system, are free from coma and astigmatism by virtue of the diaphragm disposed in the vicinity of the center of curvature of the concave mirror. However, since the curvature of field cannot be corrected, a large field curvature occurs.
U.S. Reissued Pat. No. 27,356 discloses an ocular optical system which uses a semitransparent concave mirror and a semitransparent plane mirror, each as a single independent constituent element, to project an object surface in a distance. However, since the semitransparent concave mirror and the semitransparent plane mirror are each formed from a single independent constituent element, when the field angle and the pupil diameter are enlarged, off-axis aberrations such as field curvature, coma, etc. are likely to occur.
The causes of field curvature and comatic aberration in the optical system of U.S. Reissued Pat. No. 27,356 will be explained below with reference to FIG. 19. In the figure, the semitransparent plane mirror is denoted by reference numeral 1, and the semitransparent concave mirror is denoted by reference numeral 2. As shown in the figure, a bundle of light rays passes through or is reflected by each surface in the sequence: 1-1, 1-2, 2-1, 1-3, 2-2, and 2-3. At the surfaces 1-1, 1-2, 2-2 and 2-3, the ray bundle is subjected to refracting action; at the surfaces 2-1 and 1-3, the ray bundle is subjected to reflecting action. Since no optical element having power is disposed in front of the semitransparent plane mirror 1, neither field curvature nor comatic aberration is produced by the refracting action of the semitransparent plane mirror 1. Further, since the semitransparent plane mirror 1 is an optical element having no power, neither field curvature nor comatic aberration is produced by the reflecting action of the surface 1-3. Since each surface of the semi-transparent concave mirror 2 has power, both field curvature and comatic aberration are produced by the reflecting action of the surface 2-1 and the refracting action of the surfaces 2-2 and 2-3. However, since the semitransparent concave mirror 2 is a meniscus lens whose surfaces have the same power, the surfaces 2-2 and 2-3 produce field curvatures which are opposite in sign but approximately equal in quantity to each other, and also produce comatic aberrations which are opposite in sign but approximately equal in quantity to each other. Accordingly, the field curvatures cancel each other, and so do the comatic aberrations. Therefore, the surfaces 2-2 and 2-3 do not contribute to the occurrence of field curvature and comatic aberration at the image plane. In other words, the main causes of field curvature and comatic aberration at the image plane are positive field curvature and positive comatic aberration produced by the reflecting action at the surface 2-1.
As the field angle increases, the power of each surface of the semitransparent concave mirror 2 also becomes stronger, resulting in an increase of the amount of positive field curvature produced at the surface 2-1. As the pupil diameter increases, the difference in incident angle between extra-axial upper and lower marginal rays at the surface 2-1 increases, resulting in an increase of the amount of positive comatic aberration produced.
As has been described above, it is difficult for an optical system consisting of a semitransparent plane mirror and a semitransparent concave mirror to correct both field curvature and comatic aberration satisfactorily, and hence difficult to cope with the demand for achievement of a wide field angle and a large pupil diameter.